Cloth simulation glitch
(Answer copied from Self Collision - Blender Cloth Sim shrinks mesh as can't flag as duplicate since it has no upvotes...)
Most likely your cloth vertices are self-colliding with their neighbours due to the Self Collision Distance being too large.
Firstly, when running a simulation always ensure the meshes are created at 'real world' scales - eg, for a bedsheet you might assume 2 metres a side (or 2 Blender Units). Ensure you Ctrl+A and 'Apply Scale' following any changes to scale - to ensure the simulation is using the correct scale.
Secondly, reduce the Self Collisions 'Distance' property. At Blender 2.83.2 this defaults to 0.015 metres (1.5cm). If your vertices are closer together you may want to reduce that value.